Audio frequency amplifier



A ril 5, 1938. R. A. BIERWIRTH AUDIO FREQUENCY AMPLIFIER Filed Oct. 31, 1933 IOIOO FE souswcv Patented Apr. 5, 1938 UNITED STATES PATENT OFFICE AUDIO FREQUENCY AIVIPLIFIER tion of Delaware Application October 31,

12 Claims.

The present invention relates to audio-frequency amplifiers. More particularly, the present invention relates to tone filter systems for amplifiers of that character, and has for one 5 object the provision of a simplified and inexpensive combined tone filter and output transformer for an audio-frequency amplifier.

A further object of the present invention is to provide an improved and simplified tone control system for an audio-frequency amplifier.

A still further object of the invention is to provide an improved audio-frequency amplifier wherein undesirable high audio-frequency interference and higher audio-frequency harmonics generated in the amplifier devices, may be filtered from the output of the amplifier following the power amplifier devices or tubes.

It is generally undesirable to extend the audiofrequency response of a radio receiver above approximately 5000 cycles, because the increase in extraneous noise and heterodyne sounds more than outweighs the improvement in fidelity of reproduction gained in extending the response range above that value. In accordance with the invention, this high frequency cut-off is acquired by means of a filter incorporated in the output transformer of the amplifier following the last amplifier tube or tubes, whereby the filter means is simplified and at the same time, the higher harmonic signals, generated in the tubes, are eliminated at the output end of the amplifier.

The invention will, however, be better understood when taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing, Figure 1 is a schematic circuit diagram of an audio-frequency amplifier embodying the invention; Fig. 2 is a curve diagram illustrating certain operating characteristics of the amplifier shown in Fig. 1, and Figs. 3, 4, and 5 are circuit diagrams similar to Fig. 1., illustrating modifications of the invention as applied to an amplifier of that character.

Referring to Fig. 1, B and l are electric dis.- charge devices arranged in push-pull relation to each other as a power or output amplifier stage for an audio-frequency amplifier, and for this purpose, are provided with a balanced or pushpull input transformer 8 and a balanced or pushpull output transformer 9.

The output stage supplies signals to a suitable sound output device such as a loudspeaker l0 connected with a secondary winding ll of the output transformer 9. The balanced primary winding 12 is connected to the balanced anode 1933, Serial No. 696,002 (01. 1'l844) of circuit l3 of the devices 6 and I and to a source of anode potential as indicated by the supply lead 14.

The balanced input circuit for the devices 6 and I is indicated at l5 and is connected with the balanced secondary l6, one half of which is provided also with a suitable tone control means in shunt thereto, comprising series connected condenser ll and variable resistor Ill.

The output stage may be driven from any suitable audio-frequency signal source, such as a preceding amplifier stage comprising an electric discharge amplifier device I9 connected through its output circuit Zll-With the primary winding 2| of the transformer 8, and provided with a suitable input transformer 22, as indicated.

Audio-frequency signals supplied to the terminals of the amplifier indicated at 23 are amplified through the first stage amplifier device 19 and balanced output stage provided by the devices 5 and l and are delivered to thesound producing means 58 through the output transformer 9. Thus, all signals including undesired high frequency disturbances and the higher harmonies generated in the amplifier devices shown, and in preceding devices and circuits not shown, which may be connected with the terminals 23, are required to pass through the transformer 9.

In accordance with the invention, such undesirable signals and harmonics are caused to be filtered within the transformer through the medium of an auxiliary or tertiary winding 24, across which is connected a tuning capacitor 25, which serves to tune the leakage reactance of the tertiary winding to the desired cut-off frequency.

Referring, now, to Fig. 2, assuming an amplifier frequency response characteristic as indicated by the dotted curve 26 plotted with respect to frequency as indicated and the ratio frequency range, whereby the amplifier is capable of reproducing signals and transmitting said signals to the output circuit throughout a signal range in which interference and harmonics may be encountered in the operation of the amplifier.

By providing a tertiary winding 24 adjacent to the secondary winding II in the output transformer and tuning the leakage reactance of this winding as above described, the characteristic 26 may be made to assume a form as indicated by the curve 21 whereby the higher frequency interfering signals and harmonics are greatly attenuated.

It will be noted from the shape of the curve 27, that the cut-off is comparatively sharp and that it rises beyond the lowest point of the dip of the curve. The broadness of the dip in the frequency characteristic or broadness' of the band of attenuation, is governed by the relative values of the capacitor, the leakage reactance of the tertiary winding, and the circuit impedance.

The clip in the characteristic curve occurs at the frequency where the leakage reactance of the tertiary winding resonates with the shunt tuning capacitor. This causes the winding with which the tertiary winding is associated to operate as if a low impedance shunt path was connected across it, at the resonant frequency, thereby reducing the response to a low value in a predetermined audio-frequency range, such as a higher frequency range including interfering signals,

noises and the like.

Instead of connecting the capacitor directly across the tertiary winding, as shown in Fig. 1, it may be connected across the tertiary and part or all of either of the other windings connected in series, as shown in Fig. 3, wherein the output winding H for the reproducer I 0 is connected in series with the tertiary winding 24 through a resonating condenser 28 and a series connection 29. With this arrangement the dip in the characteristic will be broader than that obtained with the arrangement of Fig. 1.

A plurality of additional tertiary windings may be provided on the output transformer as indicated in Fig. 4 at 30 and 3!, one being adjacent to the primary winding and the other being adjacent to the secondary winding. Each of the windings is connected to a shunt capacitor 34 and tuned thereby to different signal frequencies. In this way, a very sharp cut-off and broad band of attenuation may be obtained as shown by the curve 36 in Fig. 2.

By varying the capacity or the inductance in the tertiary circuit or the resistance of the circuit, the high frequency cut-off characteristic and therefore the tone of the signals reproduced by the device I0 may be varied. Referring to Fig. 5, a tone control means and circuit in connection with a single tertiary winding for the output transformer is shown.

The tertiary winding indicated at 31 is provided with a plurality of taps 38 and is connected in circuit through a variable tuning capacitor 39 and a variable resistor 40 whereby the tuning or impedance characteristic of the tertiary circuit may be varied thereby to vary the cut-off charactertistic of the output circuit 4| between that indicated at 21 and that indicated at 25.

It is obvious that a selected one of the variable circuit components may be utilized entirely as the tone control means, while the remaining elements are fixed at predetermined values, and, in the preferred arrangement, the variable resistor 40 is employed as the variable tone control means, thereby simplifying the transformer winding and the condenser and reducing the cost.

From the foregoing description, it will be seen that a combined filter and output transformer is provided for an audio-frequency amplifier,

having a cut-off characteristic for eliminating, at the end of the audio-frequency channel, all undesired high frequency disturbances both in the signal and as generated in the amplifying apparatus. A filter arrangement of this type is simpler and may be provided at lower cost than an equivalent external filter requiring additional mounting space and mounting means. Furthermore, the shunt tuning capacitor may be reduced in size by increasing the number of turns on the tertiary winding so that it may be made as small as is practical and economical.

A further advantage, hereinbefore noted, lies in the fact that the filter means, being a part of the output transformer, thus follows the power tubes and other amplifying apparatus in the signal channel whereby the higher harmonics generated in the tubes are eliminated at the output of the channel and just preceding the sound producing or other output device.

By way of example, a combined output filter and transformer which has been found to be satisfactory in connection with low power amplifiers such as are used in battery operated receivers, is provided by a tertiary winding of substantially 3800 turns placed on top of the secondary winding of the output transformer between a class B output stage and a dynamic loud speaker. A .0024 microfarad capacitor connected in shunt thereto tunes this winding to substantially 6000 cycles at the middle of the dip sufliciently to eliminate a larger portion of the objectionable high frequency response at and above that frequency.

I claim as my invention:

1. The combination with an audio-frequency power amplifier stage, of an output transformer therefor having an output winding, and an auxiliary winding having a leakage reactance tunable within the audio-frequency range, and capacity means connected in circuit with said auxiliary winding to tune said leakage reactance thereof to a cut-off frequency at the higher frequency end of the audio-frequency response range of said amplifier stage.

2. The combination with an audio-frequency power amplifier stage including a power output tube, of a combined tone filter and output transformer therefor having a primary winding, a secondary winding adapted in impedance to be connected to the voice coil of a sound producing device, a tertiary winding having a predetermined leakage reactance tunable within the audio-frequency range and a tuning capacitor connected in circuit with said last named winding for resonating with said leakage reactance in a predetermined higher audio-frequency range including higher frequency tube, static and circuit noises, thereby to cut off electrically said interference noises at one point adjacent to said voice coil connection.

3. The combination with an audio-frequency power amplifier stage, of an output transformer therefor having coupled primary and secondary windings, and at least two auxiliary windings, said windings having predetermined leakage reactance tunable within the audio-frequency range, and separate means connected in parallel with each of said auxiliary windings, to tune the leakage reactance within a predetermined wide portion of said audio-frequency range.

4. The combination with an audio-frequency power amplifier stage, of an output transformer therefor having coupled primary and secondary windings, and at least two auxiliary windings,

said windings having a predetermined leakage reactance tunable within the audio-frequency range, and separate means connected in parallel with each of said auxiliary windings, to tune the leakage reactance within a predetermined relatively wide portion of said audio-frequency range, one of said auxiliary windings being adjacent to the primary winding and another of said auxiliary windings being adjacent to the secondary Winding.

5. The combination with an audio-frequency power amplifier stage, of an output transformer therefor which comprises a primary winding, a secondary winding, and a tertiary winding having a predetermined leakage reactance tunable to an audio frequency, said tertiary winding and one of said other windings being connected in series, and a tuning condenser connected across said series connected windings to tune the leakage reactance to resonance within a predetermined higher audio-frequency range.

6. The combination with an audio-frequency power amplifier stage, of an output transformer therefor having an auxiliary winding, said winding having a predetermined leakage reactance tunable within the audio-frequency range, means connected in parallel relation with said winding to tune said leakage reactance within a predetermined portion of said audio-frequency range and means for varying the impedance of the circuit provided by said parallel connected winding and tuning means.

'7. The combination with an audio-frequency power amplifier stage, of an output transformer therefor having an auxiliary winding, said winding having a predetermined leakage reactance tunable within the audio-frequency range, means connected in parallel relation with said winding to tune said leakage reactance within a predetermined portion of said audio-frequency range, and a variable resistor in circuit between said auxiliary winding and the tuning means therefor, for varying the audio-frequency response characteristic of said amplifier.

8. The combination as defined in claim 1 further characterized by the fact that the amplifier stage comprises. two power amplifier tubes connected in balanced relation to each other and that the output transformer is a balanced transformer connecting said tubes in said balanced relation to each other.

9. In an audio-frequency amplifier, the combination with a balanced output stage therefor,

of a signal output transformer having a balanced primary winding, a secondary winding adapted to be connected with a loudspeaker device for supplying amplified signals thereto, an output circuit connected with said secondary winding, an auxiliary winding for said trans.- former having a predetermined leakage reactance tunable within the audio-frequency range, said last-named winding having a plurality of taps, and a control circuit connected between one terminal of said winding and one of said taps for controlling the frequency cut-off characteristic of the output circuit.

10. In an audio-frequency amplifier, the combination with a balanced output stage therefor, of a signal output transformer having a balanced primary winding, a secondary winding adapted to be connected with a loudspeaker device for supplying amplified signals thereto, an output circuit connected with said secondary winding, an auxiliary winding for said transformer having a predetermined leakage reactance tunable within the audio-frequency range, said lastnamed winding having a plurality of taps, a control circuit connected between one terminal of said winding and one of said taps for varying the frequency cut-off characteristic of the output circuit, and means providing a variable impedance in said control circuit.

11. An audio-frequency amplifier for radio receiving apparatus and the like having a final output transformer, characterized by the fact that it is provided with an auxiliary winding having a leakage reactance tunable by a single condenser of relatively low capacity and size, whereby the transformer operates as an output coupling device and as a final high frequency noise cutoff device for the system to which the amplifier is connected.

12. In an audio frequency signal amplifying channel, the combination with an audio-frequency power amplifier stage, of an output transformer at the output end of said channel and following said stage, said transformer having an output winding and an auxiliary winding having a leakage reactance tunable within the audiofrequency range, and means providing capacity connected in circuit with said auxiliary winding to tune said leakage reactance thereof to a cutoff frequency at the higher frequency end of the audio-frequency response range of said amplifying stage.

RUDOLPH A. BIERWIRTH. 

